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Title Functional Analysis of RIG-I and RNP Complexes in theAntiviral Interferon System( Dissertation_全文 )
Author(s) Oh, Seong-Wook
Citation Kyoto University (京都大学)
Issue Date 2016-05-23
URL https://doi.org/10.14989/doctor.k19907
Right
Type Thesis or Dissertation
Textversion ETD
Kyoto University
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Functional Analysis of RIG-I and RNP Complexes
in the Antiviral Interferon System
Oh Seong-Wook
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Contents
Page
Abstract ···························································································· 1
1. Introduction ·················································································· 3
1-1. The innate immunity 3
1-2. RIG-I-like receptors 4
1-2.1. Structure 4
1-2.2. Signaling 5
1-2.3. Detection 8
1-2.4. Ligands 9
1-3. Stress granules 13
1-4. ssRNA(-) viruses: structure and replication 15
1-5. Aim of this study 18
2. Materials and Methods ·································································· 20
2-1. Cells 20
2-2. Viruses and infection 20
2-3. Arsenite, nocodazole, and RNase treatments 21
2-4. Immunofluorescence 21
2-5. RNA-FISH 22
2-6. Computer-based statistical cell analysis 23
2-7. RNAi gene knockdown 23
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2-8. Western blotting 24
2-9. Reverse transcription and quantitative PCR (RT-qPCR) 25
2-10. RNA preparation and transfection 25
2-11. In vitro transcription 26
2-12. Northern blotting 27
2-13. Strand-specific RT-qPCR 28
3. Results ······················································································· 30
3-1. NDV infection induced formation of viral granules and avSGs 30
3-2. NDV vRNAs were derived from vRCs and vRNA(+) migrated to avSGs 30
3-3. NDV vdsRNA production was retained in vRCs 35
3-4. NDV vRCs formation was synchronized with primary IFNB induction 35
3-5. RIG-I accumulated in both NDV vRCs and avSGs 38
3-6. IPS-1 associated with both NDV vRCs and avSGs 43
3-7. Dissociation of IPS-1 from NDV vRCs/avSGs impaired IFNB induction 43
3-8. avSGs was required for full activation of IFNB gene expression 46
3-9. Viral poly(A)+ RNA stimulated IFNB induction 47
3-10. NDV poly(A)+ RNA formed double-stranded, 5’-triphosphate structure 52
3-11. NDV produced read-through transcript 56
3-12. NDV, RSV, and VSV produced leader-containing read-through RNA 58
3-13. In vitro Le-N RNA induced IFNB gene expression SGs formation 64
4. Discussion ··················································································· 67
4-1. vRCs; a locale where RIG-I recognizes its authentic ligand, vgRNA 67
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4-2. avSGs; a locale where RIG-I encounters sequestered viral poly(A)+ RNA 68
4-3. avSGs; a critical platform for the RIG-I sensing of multiple viral RNAs 69
4-4. The viral read-through transcripts; a novel natural ligand of RIG-I 70
4-5. Conclusion 71
5. References ·················································································· 74
6. Acknowledgements ········································································ 88
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Abstract
RIG-I is a crucial cytosolic viral RNA sensor that triggers signal transduction to
induce the antiviral cytokine, interferon (IFN). RIG-I selectively recognizes non-self
signatures of viral RNA, such as 5’-triphosphate and double-stranded RNA. However,
the spatiotemporal dynamics of RIG-I recognition and its signaling remain elusive. In
this study, I intensively investigated the time-course of events in cells infected with
NDV, a negative-single-stranded RNA virus of Mononegavirales, especially focusing on
the correlation between the action of RIG-I and the kinetics of IFN induction.
In the early phase of NDV infection, formation of viral replication complexs (vRCs)
consisting of viral proteins and RNAs was observed, and RIG-I accumulated in vRCs to
induce primary IFN. In the late phase, host antiviral stress granules (avSGs) were also
induced beside vRCs, and RIG-I was also distributed to avSGs. Interestingly, only
positive-strand poly(A)+ viral RNA migrated to avSGs among viral RNAs in vRCs.
Inhibition of avSGs considerably impaired the magnitude of IFN upon viral infection,
suggesting that avSG-associated RIG-I was responsible for the secondary amplification
of IFN. I further identified uncapped poly(A)+ viral RNA covering leader and N gene
derived from read-through transcription of NDV as the responsible ligand for RIG-I
activation. Intriguingly, production of this read-through RNA was not limited to NDV
but commonly observed in other Mononegavirales viruses, RSV and VSV.
Taken together, this study demonstrates that the clusters of RNP complexes; vRCs
and avSGs are locales where RIG-I encounters viral RNAs to trigger primary induction
and secondary amplification of IFN, and identifies a viral read-through transcript as a
novel natural ligand of RIG-I.
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1. Introduction
1-1. The innate immunity
Higher animals, including humans, have developed a self-defense system against
infection by pathogenic microbes such as fungi, parasites, bacteria, and viruses. The
system consists of two principal processes; “innate immunity” and “adaptive immunity”,
both of which are indispensable for controlling cellular responses against infection. The
innate immunity, in particular, governs rapid clearance of invading pathogens
immediately after infection. Therefore, how animals build this “first barrier of defense”
against infection is a critical determinant of their fate. Pathogens, on the other hand,
have evolved to subvert the host’s barrier by disrupting key components of the defense
system for their successful survival, indicating the prominent role of the innate
immunity on host-pathogen interaction.
The greatest advance in the study of the innate immunity was the discovery of host
pattern recognition receptors (PRRs), which detect a diverse range of conserved
pathogen-specific structures referred to as pathogen-associated molecular patterns
(PAMPs) [1]. Based on genetic and functional relationships, PRRs are now classified
into four large groups; Toll-like receptors (TLRs), C-type lectin receptors (CLRs),
nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), and retinoic
acid-inducible gene I (RIG-I)-like receptors (RLRs). TLRs were first identified as a
human homolog of the Drosophila Toll protein, and now 10 human TLR family
members, which are all expressed either on the cell membrane or on the endosomes of
immunocompetent cells such as macrophages and dendritic cells, have been
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characterized [1]. TLRs encounter PAMPs exclusively in the extracellular compartment
and elicit multiple cellular programs including inflammation, apoptosis, and stimulation
of downstream adaptive immunity. CLRs are also known as transmembrane PRR, which
plays more important role in antifungal innate immunity.
Meanwhile, PAMPs exposed in the cytoplasm should be detected by other
intracellular PRRs. NLRs, which are composed of 22 members in humans, are
specialized PRRs for bacterial PAMPs recognition leading to activation of inflammatory
and apoptotic responses. Intracellular recognition of viral single-stranded RNA (ssRNA)
and double-stranded RNA (dsRNA) is mediated by RLRs, which consist of three RNA
helicases including RIG-I, MDA5, and LGP2. RIG-I and MDA5 have been
demonstrated to directly bind to a variety of viral RNA species and trigger a signaling
cascade to induce production of antiviral cytokine termed interferon (IFN). TLRs, NLRs,
CLRs, and RLRs are all critical PRRs that synergistically drive the innate immune
responses against microbial infection [2].
1-2. RIG-I-like receptors
1-2-1. Structure
Retinoic acid-inducible gene I (RIG-I), which is encoded by the DDX58 gene, is the
DExD/H-box RNA helicase expressed in the cytoplasm. RIG-I was first identified as a
crucial sensor molecule that detects viral infection and induces an important antiviral
cytokine, Type I IFN (IFN-α/β) [3]. RIG-I consists of three functional domains;
N-terminal tandem caspase activation and recruit domains (CARDs), intermediate
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DExD/H-box RNA helicase domain, and C-terminal domain (CTD) (Figure 1-1). The
helicase domain includes an ATP-binding site, which is essential for the
helicase/ATPase activity that drives RIG-I. RIG-I shares similar domain structures and
functional properties with melanoma differentiation-associated gene 5 (MDA5) and
laboratory of genetics and physiology 2 (LGP2), which thereby gathers these proteins
into a family of RNA helicases referred to as RIG-I-like receptors (RLRs) (Figure 1-1).
RLRs are found only in the higher vertebrates, and are ubiquitously expressed in most
tissues. Among RLRs, RIG-I and MDA5 equip all three functional domains and are
defined as positive regulators of antiviral response. On the other hand, since LGP2 lacks
the CARDs, it is supposed to play distinct roles in RLR signaling (see below).
1-2-2. Signaling
In a steady state without viral infection, RIG-I is kept in its “closed” form by the
covering of the effector domain CARDs with the helicase domain, hence it is inactivate.
Upon viral infection, RIG-I directly binds to viral RNA released into the cytoplasm via
the helicase domain and CTD, and then undergoes an ATP-dependent conformational
change to take its “open” form (Figure 1-2). Activated RIG-I exposes the CARDs which
TRIM25 then modifies with covalent K63-polyubiquitin linkage, and then it forms
filamentous oligomerization along with viral RNA [4-6]. Subsequently, RIG-I associates
with a downstream CARD-containing adaptor protein termed IFN-β promoter
stimulator-1 (IPS-1) (also known as MAVS, VISA, or Cardif), which is anchored to the
mitochondrial outer membrane [7-10]. Upon association, IPS-1 forms a prion-like
aggregation on the membrane [11] driven by the optimal regulation of mitochondrial
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fusion and fission by MFN1 and OPA1 [12]. The CARD-CARD interaction of RIG-I
with IPS-1 serves as a scaffold for the assembly of a protein complex consisting of
TRAF3/6 [9, 13], caspase-8/10 [14], RIP1 [7], and TRADD [15]. Formation of the
complex further recruits kinase complexes, such as TBK1/IKK-I [16, 17] and
IKKα/IKKβ/IKKγ [18-22] leading to activation of transcription factors, IRF-3/7 and
NF-κB. These transcription factors in turn translocate into the nucleus and activate the
genes coding for Type I and III IFNs (IFN-α/β and IFN-λ) [23] and a set of
Figure 1-1. Domain structure of RLRs and IPS-1
RIG-I and MDA5 consist of N-terminal tandem CARDs, intermediate DExD/H-box RNA
helicase domain, and C-terminal domain (CTD). LGP2 lacks the CARDs. A critical residue
for the ATP binding activity of RIG-I is shown by the yellow star (K270). IPS-1 consists of
an N-terminal single CARD, proline-rich domain (PRD), and C-terminal transmembrane
domain (TMD). IPS-1 is anchored to the mitochondrial outer membrane via TMD.
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Figure 1-2. RIG-I-driven IFN signaling pathway
Upon viral infection, RIG-I binds to viral RNA via the helicase domain and CTD, and
undergoes ATP-dependent conformational change to take active form. Exposed RIG-I
CARDs are oligomerized and then interact with the CARD of adaptor protein, IPS-1 that
aggregates on the mitochondrial outer membrane. Consequently, a set of kinases such as
TBK-1/IKK-i and IKKα/IKKβ/IKKγ are recruited and activate transcription factors,
IRF-3/7 and p65/p50 (NF-κB). These transcription factors translocate to the nucleus and
activate gene transcription for IFNs cooperating with CBP/p300. Produced IFNs are
secreted from the cells, and bind to IFN receptors expressed on the surface of neighboring
cells. Finally, the JAK-STAT signaling pathway is activated and hundreds of ISGs are
expressed for antiviral activities.
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proinflammatory cytokines in collaboration with co-activator, CBP/p300 [24]. Produced
IFNs are secreted from the infected cells and binds to cognate receptors expressed on
both IFN-producing cells and neighboring cells. Eventually, the JAK-STAT signaling
pathway is activated to induce the expression of hundreds of IFN-stimulated genes
(ISGs), which encode a variety of antiviral proteins. These proteins also include
signaling molecules such as RIG-I and IRF-7, thereby IFN production is positively
amplified for intense antiviral activity.
1-2-3. Detection
Among RLRs, RIG-I and MDA5 share extremely similar domain structures and
signaling pathways, however, their recognitions are quite different; suggesting a
non-redundant role of RLRs in viral detection (Table 1-1). RIG-I detects a wide variety
of RNA viruses such as Sendai virus (SeV), Newcastle disease virus (NDV), respiratory
syncytial virus (RSV), vesicular stomatitis virus (VSV), influenza A virus (IAV), and
hepatitis C virus (HCV), which all belong to negative-sense ssRNA (ssRNA(-)) virus
families [25-28]. On the other hand, MDA5 exclusively recognizes viruses belonging to
a positive-sense ssRNA virus family, Picornaviridae, such as encephalomyocarditis
virus (EMCV), Mengo virus, coxackie virus, and enterovirus [29-31]. Vaccinia virus
(VACV), a DNA virus belonging to Poxviridae, is recognized by MDA5 [32, 33];
however, the myxoma virus of this family is recognized by RIG-I [34]. Moreover, both
RIG-I and MDA5 are often required for the recognition and sufficient antiviral response
against certain types of viruses such as Semliki Forest virus [35], measles virus [36],
West Nile virus [37, 38], dengue virus [37], and reovirus [37, 39]. It is quite possible
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that these differential roles in the recognition of viral infections are due to the distinct
structures of viral genomes. LGP2, on the other hand, lacks the CARDs unlike RIG-I
and MDA5. Therefore, LGP2 was assumed to play a more negative role in the IFN
system; however, its positive regulatory activity in antiviral response against certain
viruses (e.g. NDV, VSV, and EMCV) has been reported [40].
1-2-4. Ligands
The properties of RNA ligands for RIG-I and MDA5 have been extensively
explored by means of artificially synthesized RNAs (Table 1-2). Both RIG-I and MDA5
have been shown to be activated by a synthetic dsRNA, poly(I:C), and induce a strong
IFN response [3, 39]. Nonetheless, relatively short poly(I:C) (<300 bp) was
preferentially recognized by RIG-I whereas long poly(I:C) (>4 kbp) was an exclusive
substrate of MDA5; suggesting that the length of dsRNA is an important determinant of
the ligand specificity of RIG-I and MDA5 [39].
Along with dsRNA, in vitro-transcribed ssRNA by phage RNA polymerase, such as
T7-Pol, was also reported to be a good inducer of IFN. In vitro transcripts contain a
5’-triphosphate, and this signature appeared to be a critical determinant of non-self
recognition by RIG-I [41]. This seems to be a rational strategy to discriminate between
non-self and self RNA because the 5’-triphosphate moiety of the host RNA is removed
or masked by the RNA processing. Viruses belonging to Picornaviridae evade RIG-I
detection because their 5’-triphosphate is masked by covalently attached viral protein,
Vpg [42, 43]. The in vitro transcripts, in addition, present short base-paired structure (i.e.
double-stranded) at the terminus by the “copy-back” mechanism. This structure was
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also demonstrated to be a potent enhancer of the stimulatory 5’-triphosphate ssRNA [44,
45]. Indeed, the genome of IAV contains 5’-triphosphate and partial double-stranded
structure (>15 bp) termed the “panhandle”, thereby being a natural ligand of RIG-I [28].
Likewise, SeV generates defective-interfering (DI) viral genomes with a hairpin-like
double-stranded structure (100-1,000 bp), and this “snap-back” structure likely
represents a ligand of RIG-I [46]. Of note, not only viral genomes but also replicative
intermediate RNAs are capable of presenting a dsRNA structure. A variety of ssRNA
viruses, which are partially recognized by RIG-I, have been reported to produce
substantial amounts of cytosolic viral dsRNA during their life-cycle [47, 48]. Recently,
it was shown that RIG-I also recognize base-paired 5’-diphosphate RNA species, and
this recognition is essential for responding to reovirus infection [49]. In addition to the
structural specificity, sequence dependency of RIG-I recognition has been reported.
Activation of RIG-I in response to HCV infection required poly(U/UC) tract at its
3’-UTR along with 5’-triphosphate [27]. Furthermore, the AU-rich sequence in IAV
3’-UTR was recognized by RIG-I, however, 5’-triphosphate moiety was dispensable
[50]. Multiple determinants of the non-self RNA recognition of RIG-I are being
considered.
Concerning MDA5 ligands, genomes of EMCV and VACV form high molecular
weight complex interlaced with ssRNA and dsRNA [32]; and this “RNA web” structure
was considered to be involved in MDA5 activation. However, neither the precise
structure of the complex nor its recognition mechanism by MDA5 is still enigmatic.
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Table 1-1. Viruses recognized by RIG-I and MDA5
RLRs Viruses Families; Genome types References
RIG-I
Sendai virus
Newcastle disease virus
Respiratory syncytial virus
Paramyxoviridae;
ssRNA(-), non-segmented
Yoneyama, Kato et al. (2005)
Kato et al. (2005)
Loo et al. (2008)
Rabies virus
vesicular stomatitis virus
Rhabdoviridae;
ssRNA(-), non-segmented
Hornung et al., 2006
Yoneyama, Kato et al. (2005)
Rift Valley fever virus
La Crosse virus
Bunyaviridae;
ssRNA(-), non-segmented
Weber et al. (2013)
Weber et al. (2013)
influenza A virus
influenza B virus
Orthomyxoviridae;
ssRNA(-), segmented
Kato et al. (2006)
Loo et al. (2008)
hepatitis C virus
Japanese encephalitis virus
Flaviviridae;
ssRNA(+), non-segmented
Saito et al. (2007)
Kato et al. (2006)
Epstein-Barr virus Herpesviridae;
dsDNA Samanta et al. (2008)
myxoma virus Poxviridae;
dsDNA Wang et al. (2008)
MDA5
encephalomyocarditis virus
Mengo virus
coxackie virus
enterovirus
Picornaviridae;
ssRNA(+), non-segmented
Kato, Gitlin et al. (2006)
Kato et al. (2006)
Kato et al. (2006)
Feng et al. (2012)
vaccinia virus Poxviridae;
dsDNA Pichlmair, Delaloye et al. (2009)
RIG-I
+
MDA5
measles virus Paramyxoviridae;
ssRNA(-), non-segmented Ikegame et al. (2010)
Semliki Forest virus Togaviridae;
ssRNA(-), non-segmented Schulz et al. (2010)
West Nile virus
Dengue virus
Flaviviridae;
ssRNA(+), non-segmented
Loo et al. (2008)
Loo et al. (2008)
Reovirus Reoviridae;
dsRNA Kato, Loo et al. (2008)
ssRNA: single-stranded RNA, dsRNA: double-stranded RNA, dsDNA:
double-stranded DNA, (-): negative-sense genome, (+): positive-sense genome
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Table 1-2. Structures of RIG-I and MDA5 ligands
RNA types Structures RLRs References
short Poly(I:C)
dsRNA >300 bp RIG-I Kato et al. (2008)
long Poly(I:C)
dsRNA >4 kbp MDA5 Kato et al. (2008)
in vitro T7 transcript
ssRNA (copy-back) RIG-I
Schlee et al. (2009)
Schmidt et al. (2009)
IAV genome
ssRNA (panhandle)
RIG-I Rehwinkel et al.(2010)
SeV genome
ssRNA (DI, snap-back) RIG-I Strähle et al. (2007)
HCV genome
ssRNA (homopolymer) RIG-I Saito et al. (2008)
EMCV genome
ss/dsRNA (RNA web)
MDA5 Pichlmair et al. (2009)
ssRNA: single-stranded RNA, dsRNA: double-stranded RNA
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1-3. Stress granules
Cells face a lifetime risk of being exposed to external stresses from their
environment. Consequently, cells connaturally equip intrinsic anti-stress strategies to
survive from such unfavorable circumstances. These stresses include, for instance, UV
exposure, heat shock, oxidation, ER stress, and starvation. Viral infection is also a
pivotal stressor that stimulates cells to execute an anti-stress response. Upon these stress
signals, four protein kinases, HRI, PERK, GCN2, and dsRNA-inducible protein kinase
R (PKR) are activated to phosphorylate an α-subunit of eIF2 (eIF2α) (Figure 1-3).
Subsequently, cells provisionally interrupt the translational machinery in order to avoid
excrescent protein synthesis and shelter accumulated mRNA by sequestering them in
cytoplasmic RNA-protein (RNP) complexes, termed stress granules (SGs) [51]. A SG is
typically composed of 40S ribosomal subunits, a subset of translation initiation factors
(eIF2, 2B, 3, 4A, 4B, 4E and 4G), and host RNA-binding proteins such as Ras
GTPase-activating protein-binding protein (G3BP), poly(A)-binding protein (PABP),
human antigen R (HuR), T-cell intracellular antigen-1 (TIA-1), and its related protein
TIAR [51-55].
A variety of viruses have been shown to induce the formation of SGs in infected
cells [56]. SG formation is strictly regulated in a steady state, however, once cells are
exposed to viral infections, PKR is activated by viral dsRNA (vdsRNA) generated as an
intermediate product within the viral replicative life-cycle [57]. Recently, SG was
reported to play a positive role in antiviral IFN signaling against IAV lacking an
IFN-inhibitory NS1 protein (IAVΔNS1) by recruiting RIG-I and a set of antiviral host
proteins to detect the viral infection [58]. Moreover, another DExD/H-box RNA
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helicase protein DHX36 acts with PKR to induce SGs, thereby facilitates detection of
viral RNA by RIG-I [59]. These findings shed light on the function of SGs as antiviral
SGs (avSGs), a substantial platform for IFN-inducing signaling triggered by RIG-I.
Figure 1-3. Stress granule formation
Protein kinases such as HRI, PERK, GCN2, or PKR are respectively activated depending
on the type of stress signals. These kinases are dimerized and undergo autophosphorylation.
Activated kinases then phosphorylate α-subunit of eIF2 (eIF2α) and this leads to the
dissociation of the eIF2-GTP-tRNAMet
complex and the termination of functional
translation machinery. Untranslated mRNAs are then sequestered into stress granule
composed of RNA-binding proteins.
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1-4. ssRNA(-) viruses: structure and replication
ssRNA(-) viruses, which belong to Group V on the Baltimore classification, utilize
negative-sense ssRNA as their genetic materials. The ssRNA viruses are classified into
positive or negative according to the sense or polarity of their genomes. A negative
genome is further distinguished by means of its segmentation as non-segmented and
segmented. Since the negative sense viral genomic RNA (vgRNA) is complementary to
the positive sense viral messenger RNA (vmRNA), it must be transcribed into the
positive sense by RNA polymerase before translation.
Order Mononegavirales, a major subgroup of ssRNA(-) viruses, consists of four
families; Bornaviridae, Filoviridae, Paramyxoviridae, and Rhabdoviridae. Viruses
belonging to this order share an extremely similar genetic structure, and their genes
typically encodes a nucleocapsid protein (N), phosphoprotein (P), matrix protein (M),
fusion protein (F), hemagglutinin-neuraminidase protein (HN), and large
RNA-dependent RNA polymerase protein (RdRp/L). These genes are arranged on
non-segmented vgRNA with an extracistronic sequence, known as leader (Le) and
trailer (Tr), in an order corresponding to 3’-Le-N-P-M-F-HN-L-Tr-5’ [60] (Figure 1-4).
The viral RdRp first transcribes the Le sequence into positive-sense RNA without
terminal modification [61]. After Le transcription, the RdRp re-initiates transcription for
the N gene to yield N mRNA with a 5’-m7G cap and 3’-poly(A) tail. Similarly, the
polymerase synthesizes other vmRNA by transcribing downstream genes without
dissociation from the template. In later stages of the infection, polymerase switches to
replication mode in order to synthesize the entire length of antigenomic RNA (viral
complementary RNA (vcRNA)) as a template for synthesis of vgRNA. The general
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strategies of transcription and replication are similar across ssRNA(-) viruses [62]
(Figure 1-5), and the viral RNA species produced during the replication cycle are sensed
by RLRs [28, 29, 37].
Figure 1-4. Transcription of NDV gRNA
A model for transcription of a typical Mononegavirales virus, NDV, is shown. NDV genes
are arranged on negative-sense gRNA in the order 3’-Le-P-M-F-HN-L-Tr-5’ with the
intermediate IGS. The length of each sequence is indicated (nt; nucleotide). Viral RdRp (L)
first transcribes Le sequence into short positive-sense transcripts (Le(+)). After this, RdRp
starts transcription of the N gene from its GS region and terminates at the GE region
(highlighted beneath). RdRp re-initiates the next P gene transcription in the same manner,
and continues this toward the last L gene. Transcripts are all 5’-capped and
3’-polyadenylated by RdRp, except for Le(+).
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Figure 1-5. Life-cycle of ssRNA(-) viruses
After entry (step 1), vgRNA(-) is transcribed by RdRp (L) to produce vmRNA(+) (step 2)
and viral proteins are synthesized (step 3). After an adequate amount of proteins are
produced, RdRp switches to RNA replication via the synthesis of antigenomic vcRNA(+)
(step 4). Replication produces more vgRNA(-) for further transcription (white box) and
translation of vRNAs, and for assembly with the structural proteins; N, P, and L (step 5), to
produce progeny virus particles (step 6).
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1-5. Aim of this study
In the past decade or so, since RIG-I was first identified as a critical PRR for viral
RNA sensing, multitudes of studies have been carried out to explore its ultimate RNA
ligands and adversarial viruses. By grace of those challenges, we are now well-informed
about a unique architecture of the ligands within a wide variety of viruses which
activate RIG-I, and a broad composition of antiviral IFN signaling which RIG-I drives.
Nonetheless, in the context of authentic viral infection, how RIG-I spatially and
temporally encounters viruses to commence its signaling has not been fully described
thus far. In this study, I investigated the spatiotemporal action of RIG-I, especially in
line with the dynamics of viral replication, to understand a more fundamental role of
RIG-I in the viral RNA sensing and antiviral IFN system.
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2. Materials and Methods
2-1. Cells
HeLa (#CCL-2.2, ATCC), FLAG-RIG-I/HeLa (derived from HeLa; #CCL-2.2,
ATCC), FLAG-IPS-1/HeLa (derived from HeLa; #CCL-2.2, ATCC) [12],
EGFP-G3BP1/HeLa (derived from HeLa; #CCL-2.2, ATCC) [63], HEp-2 (#CCL-23,
ATCC), BHK21 (#CCL-10, ATCC) cells, and MEFs (isolated from embryos under
C57BL/6 background, Japan SLC, Inc.) were maintained in Dulbecco’s Modified
Eagle’s Medium (DMEM) (Nacalai Tesque) supplemented with 10% Fetal Bovine
Serum (FBS) (BioWest) and 1% Penicillin-Streptomycin Mixed Solution (100 U/ml
and 100 μg/ml respectively) (Nacalai Tesque).
2-2. Viruses and infection
NDV (strain Miyadera/51) was inoculated into 9-day embryonated chicken eggs and
incubated for 2 days at 37°C, followed by overnight incubation at 4°C. Allantoic fluid
containing NDV was collected from dead eggs. RSV (strain Long, ATCC VR-26) and
VSV (strain Indiana, M mutant) were propagated in HEp-2 cells and BHK21 cells
respectively, and culture supernatant was collected. The virus titer was determined by a
plaque assay using HEp-2 cells. Virus was added to cells at a multiplicity of infection
(MOI) of 1. After 1-hour incubation, the medium was replaced with fresh DMEM and
incubated for the indicated hours of infection.
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2-3. Arsenite, nocodazole, and RNase treatments
Sodium arsenite, nocodazole, and ribonuclease (RNase) A were purchased from
SIGMA-ALDRICH. ShortCut RNase III was purchased from New England Biolabs.
Cells were treated as described in the figure legends.
2-4. Immunofluorescence
Cells were fixed with 4% paraformaldehyde solution for 10 minutes at room
temperature. A 0.5% Triton X-100 solution was added to the cells for permeabilization
and incubated for 5 minutes at room temperate. Regarding blocking, 0.5 mg/ml BSA
solution in PBST (PBS containing 0.04% Tween-20) was added to the cells and
incubated for 30 minutes at room temperature. Primary antibodies were diluted in 0.5
mg/ml BSA/PBST and added to the cells, then incubated overnight at 4°C. After
washing with PBST, secondary antibodies were added to the cells at a 1:1,000 dilution
in 0.5 mg/ml BSA/PBST, and incubated for 1 hour at room temperature. After washing
with PBST, 1 μg/ml DAPI solution in PBS was added to the cells to stain the nucleus.
Cells were briefly rinsed with PBS, and then mounted with Fluromount-G
(SouthernBiotech). Images were taken by the confocal laser scanning microscope,
TCS-SP8 (Leica Microsystems). The primary antibodies used were; anti-NDV-N mouse
mAb (provided by Dr. T. Sakaguchi, Hiroshima University in Japan), anti-TIA-1 goat
pAb (#sc-1751, Santa Cruz Biotechnology), anti-TIAR rabbit mAb (#8509, Cell
Signaling Technology), anti-TIAR goat pAb (#sc-1749, Santa Cruz Biotechnology),
anti-G3BP1 mouse mAb (#sc-365338, Santa Cruz Biotechnology), anti-eIF3η
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22
(#sc-16377, Santa Cruz Biotechnology), anti-FLAG M2 mouse mAb (#F1804,
SIGMA-ALDRICH), and anti-dsRNA/J2 mouse mAb (English & Scientific Consulting
Kft.). Anti-IPS-1 guinea pig pAb was provided by Dr. I. Julkunen. Anti-RIG-I and
anti-NDV-L antibodies were originally generated by immunizing rabbits with synthetic
peptides corresponding to amino acids 793-807 of human RIG-I and 1160-1183 of
NDV-L, respectively. The secondary antibodies used were; Alexa Fluor 488 donkey
anti-rabbit IgG H+L (#A-21206), Alexa Fluor 488 donkey anti-mouse IgG H+L
(#A-21202), Alexa Fluor 488 Donkey anti-Goat IgG H+L (#A-11055), Alexa Fluor 594
Donkey anti-Rabbit IgG H+L (#A-21207), Alexa Fluor 594 Donkey anti-Mouse IgG
H+L (#A-21203), Alexa Fluor 633 Goat anti-Mouse IgG H+L (#A-21050), Alexa Fluor
633 Donkey anti-Goat IgG H+L (#A-21082), and Alexa Fluor 647 Donkey anti-Goat
IgG H+L (#A-21447), all purchased from Life Technologies.
2-5. RNA-FISH
RNA-FISH assay was performed using the QuantiGene ViewRNA ISH Cell Assay
Kit (Affymetrix) according to manufacturer’s instructions as below. Cells were fixed in
4% paraformaldehyde solution for 30 minutes and permeabilized for 5 minutes with
detergent solution. Protease solution was added to the cells at a 1:4,000 dilution in PBS
and incubated for 10 minutes. After washing with PBS, the cells were incubated with a
probe set at a 1:25 dilution for 3 hours at 40°C. The cells were further and
independently incubated with a pre-amplifier, amplifier, and label probe (all at a 1:25
dilution) for 30 minutes at 40°C. After washing with PBS, the cells were subjected to an
immunofluorescence assay. The probe sets used were; NDV-F(-) (#VF1-15407),
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23
NDV-N(+) (#VF4-15408), and human IFNB1 (#VA1-11281), all purchased from
Affymetrix.
2-6. Computer-based statistical cell analysis
Confocal micrographs of NDV-infected or arsenite-treated cells were subjected to
the automatic analysis module (Multi Wavelength Cell Scoring) of MetaMorph
Software v7.7 (Molecular Devices) in order to count vRC and SG speckles and IFNB
mRNA dots. Briefly, the total cell number was first determined by counting the number
of nuclei in the DAPI channel. A single cell area was segmented from the TIAR or
eIF3η channel in reference to the intercellular boundary of the cytoplasmic staining area.
The numbers of vRC (N), SG (TIAR or eIF3η), and IFNB mRNA were counted from
each channel.
2-7. RNAi gene knockdown
siRNAs for RIG-I/DDX58 (HSS119008), PKR (HSS108571), G3BP1 (HSS115444),
G3BP2 (HSS114988), and a negative control (#12935-300), purchased form Life
Technologies, were transfected into 1×105 HeLa cells at a final concentration of 10 nM
using Lipofectamine RNAiMAX Reagents (Life technologies). 24 hours after
transfection, the cells were transferred to new culture plates with fresh DMEM. After
being incubated for a further 24 hours, cells were subjected to the following
experiments.
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2-8. Western blotting
Cells were lysed with ice-cold NP-40 lysis buffer (50 mM Tris-HCl [pH 8.0], 150
mM NaCl, 1% NP-40, 1 mM sodium orthovanadate, 1 mM PMSF, and 0.1 mg/ml
leupeptin). After centrifugation, the supernatant was collected, mixed with an equal
volume of 2× SDS sample buffer (125 mM Tris-HCl [pH 6.8], 4% SDS, 20% glycerol,
0.01% BPB, and 10% 2-mercaptoethanol), and boiled for 5 minutes. The sample
corresponding to a protein amount of 30 μg was applied to 5-20% gradient e-PAGEL
(ATTO), separated by a standard SDS-PAGE method, and then transferred onto an
Immobilon-P PVDF membrane (MILLIPORE). The membrane was incubated in
Tris-buffered saline with 0.1% Tween-20 (TBST) containing 5% skimmed milk for 30
minutes at room temperature for blocking. The membrane was incubated with a primary
antibody diluted in the blocking buffer overnight at 4°C. After washing with TBST, the
membrane was incubated with an AP-conjugated secondary antibody diluted in the
blocking buffer for 1 hour at room temperature. After washing with TBST, protein
bands were visualized using the BCIP-NBT Solution Kit for Alkaline Phosphate Stain
(Nacalai Tesque) or ECL Prime Western Blotting Detection Reagent (GE Healthcare).
The primary antibodies used were; anti-PKR mouse mAb (#sc-6282, Santa Cruz
Biotechnology), anti-phospho-PKR rabbit pAb (#ab13447, Abcam), anti-G3BP2 goat
pAb (#sc-161612), anti-STAT1 rabbit pAb (#9172, Cell Signaling Technology),
anti-phospho-STAT1rabbit pAb (#9172, Cell Signaling Technology), and anti-β-Actin
mouse mAb (#A2228, SIGMA ALDRICH). The secondary antibodies used were; goat
anti-rabbit IgG-AP (#sc-2007, Santa Cruz Biotechnology), goat anti-mouse IgG-AP
(#sc-2008, Santa Cruz Biotechnology), anti-rabbit IgG, HRP-linked (#7074, Cell
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Signaling Technology), and anti-mouse IgG, HRP-linked (#7076, Cell Signaling
Technology).
2-9. Reverse transcription and quantitative PCR (RT-qPCR)
Total RNA was isolated from cells using TRIzol Reagent (Ambion), and treated with
RNase-Free Recombinant DNase I (Roche Diagnostics). After phenol-chloroform
extraction and ethanol precipitation, purified total RNA was subjected to cDNA
synthesis using a High-Capacity cDNA Reverse Transcription Kit (Applied Biosystems).
Gene expression levels were measured by the StepOnePlus Real-Time PCR system
(Applied Biosystems) using the TaqMan Fast Universal PCR Master Mix (Applied
Biosystems), and determined by the 2-ΔΔCt
relative quantitative method. The TaqMan
probes used for measurements were; IFNB1 (#Hs01077958_s1), ISG20 (#Hs00158122_
m1), ISG56/IFIT1 (#Hs01911452_s1), CXCL10 (#Hs01124251_g1), Ifnb1 (#Mm00439
552_s1), Isg56/Ifit1 (#Mm00515153_m1), Cxcl10 (#Mm00445235_m1), and eukaryotic
18S rRNA (#4333760F), all purchased from Applied Biosystems. The probe for NDV-N
was designed as below: 5'-GTCCGTATTTGACGAATACGAG-3' (forward primer), 5'-
CAAGGGCAACATGGTTCCTC-3' (reverse primer), and 5'-TCAGGCAAGGTGCTC-
3' (probe).
2-10. RNA preparation and transfection
Poly(A)+ mRNA was isolated from the total RNA of mock/NDV-infected (12 hpi,
MOI = 1) HeLa cells using the Oligotex-dT30 <Super> mRNA Purification Kit
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(TaKaRa) according to manufacturer’s instructions. Purification was repeated twice to
yield a pure poly(A)+ mRNA fraction. The supernatant after centrifugation was
subjected to ethanol precipitation in order to obtain a concentrated poly(A)- RNA
fraction. NDV gRNA was isolated from virus particles propagated in the embryonated
chicken eggs, as described above. Allantoic fluid was centrifuged overnight at 15,000
rpm at 4°C, and the pellet was lysed using TRIZOL Reagent (Ambion) followed by
isopropanol precipitation. 5’-triphosphate RNA was synthesized in vitro as reported
previously [64]. RNA samples were treated with 1 U of ShortCut RNase III (New
England Biolabs), 1 U of RNase-free DNase I recombinant (Roche), and 15 U of Calf
Intestine Alkaline Phosphatase (Takara) at 37°C for 30 minutes, or 10 U of Vaccinia
Capping Enzyme (New England Biolabs) according to the manufacturer’s instruction.
After this treatment, RNA samples were purified by phenol-chloroform extraction and
ethanol precipitation. Regarding RNA transfection, 200 ng of each RNA sample was
transfected into 1×105 MEFs or 2×10
5 HeLa cells using Lipofectamine 2000
(Invitrogen) according to the manufacturer’s instruction.
2-11. In vitro transcription
NDV gRNA isolated from virus particles by TRIZOL Reagent was subjected to
reverse transcription, as described above. Synthesized cDNA was further subjected to
PCR using primer sets including the T7 RNA polymerase promoter sequence (Table in
S1 Table). PCR products were in vitro transcribed by the T7 RiboMAX Express Large
Scale RNA Production System (Promega) according to manufacturer’s instructions.
Ribo m7G cap analog (Promega) and Cy3-UTP (GE Healthcare) were included in the
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reaction for 5’-m7G capping and Cy3 labeling respectively. Poly(A) Tailing Kit
(Ambion) was used for 3’-poly(A) modifications. Unincorporated nucleotides within
the samples were removed by NucAway Spin Columns (Ambion). In vitro transcribed
RNA samples were transfected, as described above.
2-12. Northern blotting
A denaturing agarose gel was prepared at a final concentration of 1% (w/v) Agarose
ME (Nacalai Tesque), 1× MESA (Dojindo), 2% formaldehyde (Nacalai Tesque), and 0.5
μg/ml ethidium bromide. A total of 250 ng of each RNA sample was mixed with an
equal volume of Gel Loading Buffer II (Ambion) and incubated at 65°C for 15 minutes,
followed by quick cooling on ice, and then electrophoresed in 1× MESA. The gel was
transferred onto a nylon membrane Hybond-N (GE Healthcare) by a capillary blotting
method using 10× SSC Buffer (Nacalai Tesque). After UV cross-linking, the membrane
was pre-incubated in PerfectHyb Hybridization Solution (TOYOBO) at 65°C for 20
minutes. An RNA probe was added to the solution and incubated at 65°C overnight. The
membrane was washed with 2× SSC (+0.1% SDS) and 0.2× SSC (+0.1% SDS) at 65°C
for 15 minutes, and then irradiated onto Storage Phosphor Screen BAS-IP (GE
Healthcare). Images were scanned using BAS-5000 Image Analyzer (Fujifilm). In order
to prepare RNA probes (N(-) and Le(-)), NDV cDNA was subjected to PCR using
primer sets including the T7 RNA polymerase promoter sequence (Table in S1 Table).
PCR products were in vitro transcribed into [α-32
P]-CTP-radiolabeled RNA probes by
Riboprobe System-T7 (Promega). Unincorporated nucleotides within the samples were
removed by NucAway Spin Columns (Ambion).
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2-13. Strand-specific RT-qPCR
Strand-specific RT-qPCR targeting viral RNA was performed as introduced
elsewhere [65]. In the RT step, cDNA complementary to the target viral RNA was
synthesized with the primer including “5’-tag”, of which sequence is unrelated to NDV,
RSV, and VSV (Tables in S2, S3, and S4 Table). After the reaction, RT sample was
treated with 10 U of Exonuclease I (New England Biolabs) at 37°C for 1 hour to
remove unincorporated primer, and then the reaction was inactivated at 60°C for 30
minutes. The tagged-cDNA was subjected to qPCR analysis with Fast SYBR Green
Mater Mix (Thermo Fisher Scientific), using a specific primer set; primer corresponding
to tag sequence and viral RNA-specific primer (Tables in S1, S2, and S3 Tables).
Standard curve was generated from ten-fold serial dilutions (1010
, 109, 10
8, 10
7, 10
6, 10
5,
104, 10
3 copies/μl) of vgRNA isolated from viral particles or in vitro synthesized viral
RNA.
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3. Results
3-1. NDV infection induced formation of viral granules and avSGs
To monitor virus infection and avSG formation, I infected HeLa cells with NDV and
detected avSG by immunostaining with antibodies to its marker proteins; TIA-1, TIAR,
G3BP1, and eIF3η (Figure 3-1). Cytoplasmic granules were not observed in uninfected
cells whereas granules containing these avSG markers were clearly detectable at 12
hours post infection (hpi). avSGs were not yet visible at 6 hpi (Figure 3-1A). To
correlate avSG formation and viral replication, I monitored the expression of viral
proteins; large polymerase (L) and nucleoprotein (N) by immunostaining (Figure 3-2).
These viral proteins clearly localized as granules detectable at 6 hpi, when avSGs
(shown by TIAR) were undetectable. At 12 hpi, avSGs were detectable as distinct
granules from viral granules. Although occasional contact between viral granules and
avSGs was observed, no merger neither co-localization was observed.
3-2. NDV viral RNAs were derived from vRCs and vRNA(+) migrated to avSGs
Viral N protein functions as an indispensable component for RNA viruses through
association with viral genomic RNA (vgRNA) for its transcription and replication. Viral
L protein is a RNA-dependent RNA polymerase that catalyzes viral transcription and
replication. Therefore, NDV viral granules containing N and L proteins were likely a
locale for viral replication. To address this further, I examined the subcellular
localization of NDV viral RNAs by RNA-FISH. At 6 hpi, negative-strand NDV viral
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B
Figure 3-1. NDV infection induced avSGs formation
(A and B) HeLa cells were either mock treated or infected with NDV (MOI = 1) for 6 and
12 hours and then immunostained for TIAR (green), G3BP1 (red), eIF3η (white in A), and
TIA-1 (white in B). Nuclei were stained with DAPI (blue). The boxed area of cell image at
12 hpi was enlarged and is displayed in the upper left of the image (A). The white scale bar
corresponds to 10 μm.
A
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RNA (vRNA(-)) corresponding to vgRNA was detected in viral granules in which N
protein localized, and this association persisted at 12 hpi (Figure 3-3A). Based on this
observation, I referred to viral granules as viral replication complex (vRC).
Positive-strand NDV viral RNA (vRNA(+)), on the other hand, was first detectable as
granules co-localized with vRCs at 6 hpi (Figure 3-3A), and interestingly, its
localization expanded beyond vRCs to avSGs (shown by TIAR) at 12 hpi (Figure 3-3B).
This observation suggested that a portion of vRNA(+) was translocated from vRCs to
avSGs by some cue. I observed no detectable background signals in mock-infected cells
by RNA-FISH (Figure 3-4).
Figure 3-2. NDV infection induced viral granules formation
HeLa cells were either mock treated or infected with NDV (MOI = 1) for 6 and 12 hours.
Cells were immunostained for L (green), N (red), and TIAR (white). The boxed area was
enlarged and is displayed in the upper left of the image. Nuclei were stained with DAPI
(blue). The white scale bar corresponds to 10 μm.
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Figure 3-3. Distribution of NDV viral RNAs
(A and B) HeLa cells were infected with NDV (MOI = 1) for 6 and 12 hours, and then
NDV vRNA(-) (red) and vRNA(+) (green) were detected by the RNA-FISH method. N
(white in A) and TIAR (white in B) were immunostained with their respective antibodies
(white). Nuclei were stained with DAPI (blue). A merged image at original magnification is
shown in the rightmost panel. The white scale bar corresponds to 10 μm.
A
B
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A
B
Figure 3-4. Specificity of RNA-FISH
(A and B) Mock-treated HeLa cells were fixed and subjected to RNA-FISH detection for
NDV vRNA(+) (green) and vRNA(-) (red). Cells were also immunostained for N (white in
A) or TIAR (white in B). Nuclei were co-stained with DAPI (blue). The white scale bar
corresponds to 10 μm.
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3-3. NDV vdsRNA production was retained in vRCs
It has been reported that certain types of RNA viruses, including NDV, produce viral
double-stranded RNA (vdsRNA) as intermediate products within their replicative
lifecycle [47, 48]. I further examined localization of NDV vdsRNA by immunostaining
with an anti-dsRNA antibody, which detects >40 bp dsRNA (ref). NDV vdsRNA was
not detectable at 6 hpi, but was clearly detectable at 12 hpi within vRCs (shown by L)
(Figure 3-5A). Interestingly, detection of vdsRNA was restricted to vRCs containing L
and vRNA(-), and avSGs containing TIAR and vRNA(+) were apparently devoid of
vdsRNA (Figure 3-5B). The antibody specificity was confirmed by loss of the reactivity
after ribonuclease digestion (RNase A and III) (Figure 3-6). Taking these observations
together, I concluded that NDV exclusively replicated within vRCs, but not within
avSGs.
3-4. NDV vRCs formation was synchronized with primary IFNB induction
To elucidate the biological significance of vRCs and avSGs in the initiation of
antiviral innate immunity, I examined the kinetics of the appearance of these granules
along with IFNB gene expression. vRCs and avSGs were monitored every 1.5 hours up
to 12 hpi by immunostaining (Figure 3-7). vRCs (shown by N) were initially detected at
4.5 hpi as small granules, whose size subsequently increased. avSGs (shown by TIAR)
were detectable as late as 7.5 hpi and persisted thereafter. Quantification analysis
revealed the temporal appearance of these granules (Figure 3-8A): vRCs-positive
cells (light gray) were first detected at 1.5 hpi and reached >90 % at 6 hpi;
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Figure 3-5. Distribution of NDV viral dsRNA
(A and B) HeLa cells were either mock treated or infected with NDV (MOI = 1) for 6 and
12 hours. (A) Cells were immunostained for L (green), vdsRNA (red), and TIAR (white).
The boxed area was enlarged and displayed on the upper left of the image. (B) NDV
vRNA(-) (red) and vRNA(+) (green) were detected by the RNA-FISH method. NDV
vdsRNA was immunostained with a specific antibody (white). A merged image at the
original magnification is shown in the rightmost panel. Nuclei were stained with DAPI
(blue). The white scale bar corresponds to 10 μm.
B
A
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avSGs-positive cells (black) were detected at 7.5 hpi and reached > 90 % at 12 hpi. It is
important to note that cells without vRCs never exhibited avSGs, except for cells treated
with arsenite (dark gray), which was used as positive control for SG formation. IFNB
mRNA accumulation was also visualized by RNA-FISH, and detection correlated well
with nuclear translocation of both IRF-3 and NF-κB (Figure 3-9). IFNB mRNA was
visible from 6 hpi and the number of IFNB-positive cells increased thereafter (Figure
3-7). This observation was also shown by quantification analysis (red + green) (Figure
3-8B). At 6 hpi, IFNB mRNA-positive cells were all vRCs-positive (red). After 7.5 hpi,
cells double-positive for vRCs and avSGs were IFNB mRNA-positive (green) and this
population increased to 66.1% at 12 hpi, suggesting that this type of cell was the main
Figure 3-6. Validation for specificity of an anti-dsRNA antibody
HeLa cells infected with NDV for 12 hours (MOI = 1) were fixed and permeabilized, and
then treated with 200 μg/ml RNase A (at low NaCl concentration) or 30 units/ml RNase III.
The cells were immunostained for L (green), vdsRNA (red), and TIAR (white). Nuclei
were stained with DAPI (blue). The white scale bar corresponds to 10 μm.
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producer of IFN-β. Interestingly, cells positive for vRCs and IFNB mRNA, but negative
for avSGs (red) retained a constant population around 3-5%. A time course of
NDV-induced IFNB mRNA expression was also monitored by RT-qPCR (Figure 3-8C),
confirming that the gene expression was first detected at 6 hpi and markedly increased
up to 12 hpi. Since IFNB mRNA was detected in cells exhibiting vRCs at 6 hpi (Figure
3-8B), vRC formation is supposed to be responsible for IFNB gene expression at 6 hpi,
a relatively early phase of infection. In the later phase, however, a population of cells
triple-positive for vRCs, avSGs and IFNB mRNA (green) dramatically increased,
suggesting that avSGs contributed to secondary IFNB gene amplification.
3-5. RIG-I accumulated in both NDV vRCs and avSGs
NDV infection is preferentially detected by the cytoplasmic viral RNA sensor, RIG-I,
a crucial molecule for IFN induction [29]. Therefore, I further examined the subcellular
localization of RIG-I in NDV-infected cells (Figure 3-10). RIG-I co-localized with
vRCs (shown by N) at 6 hpi, when avSGs (shown by TIAR) were not yet induced. At 12
hpi, its localization expanded beyond vRCs to avSGs. These results were quite
consistent with the observations that formation of vRCs and avSGs coincided with the
IFNB mRNA accumulation (Figure 3-8B) and nuclear translocation of IRF-3 and
NF-κB (Figure 3-10B).
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Figure 3-7. Time course of vRCs/avSGs formation and IFNB mRNA expression
HeLa cells were infected with NDV (MOI = 1) for the indicated time or treated with 0.5
mM sodium arsenite (ARS) for 30 minutes. After fixation, the cells were immunostained
for TIAR (green) and N (red). IFNB mRNA (white) was detected by the RNA-FISH
method. Nuclei were stained with DAPI (blue). Merge 1: nucleus, TIAR, N, merge 2:
nucleus, TIAR, N and IFNB mRNA. The yellow arrowheads are cells double-positive for
vRC and IFNB mRNA (without avSGs). The white scale bar corresponds to 10 μm.
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Figure 3-8. Quantification of vRCs/avSGs formation and IFNB mRNA expression
A B
C
(A and B) Approximately 300 HeLa cells at the indicated time points of NDV infection or
ARS treatment were counted by MetaMorph software. Cells were categorized according to
the existence of vRCs, avSGs, and IFNB mRNA as shown above. Percentages are indicated
inside the data bar. (C) HeLa cells were infected with NDV (MOI = 1) for the indicated
time up to 12 hours. Expression levels of IFNB mRNA were measured by RT-qPCR. Data
is represented as means of ±SD.
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Figure 3-9. Correlation of IFNB mRNA FISH and nuclear IRF-3/NF-κB
A
B
HeLa cells were either mock infected or infected with NDV (MOI = 1) for 12 hours. IFNB
mRNA (green) was detected by the RNA-FISH method. IRF-3 (A) and p65/RelA (B)
shown in red were immunostained with the respective antibodies. Nuclei were stained with
DAPI (blue). The white scale bar corresponds to 10 μm.
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A
Figure 3-10. RIG-I localization in NDV-infected cells
(A and B) HeLa cells were either mock treated or infected with NDV (MOI = 1) for 6 and
12 hours. Cells were immunostained for RIG-I (green), N (red), and TIAR (white) (A), or
RIG-I (green), TIAR (red), and IRF-3 (white) (B). Nuclei were stained with DAPI (blue).
The white scale bar corresponds to 10 μm. The boxed area was enlarged and displayed on
the left.
B
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43
3-6. IPS-1 associated with both NDV vRCs and avSGs
RIG-I transduces its signal to the downstream adaptor, IPS-1, which is expressed on
the mitochondrial outer membrane. I further monitored the localization of IPS-1 using
HeLa cells stably expressing FLAG-tagged IPS-1 [12] upon NDV infection. At 6 hpi,
IPS-1 localized in the proximity of vRCs, showing a yellow color in the periphery of
vRCs (as shown by L), suggesting their close interaction (Figure 3-11A). At 12 hpi,
avSGs were clearly detected and partially co-localized with IPS-1 (Fig 3-11A, bottom
right). I also confirmed the interaction of endogenous IPS-1 with both vRCs and avSGs
(Figure 3-11B). This data strongly suggests that IPS-1 was communicates with RIG-I
associated with both vRCs and avSGs for signal transduction.
3-7. Dissociation of IPS-1 from NDV vRCs/avSGs impaired IFNB gene induction
Since microtubules have been shown to drive the arrangement of mitochondria [34],
and SGs are known to translocate on the microtubule network [35], I assumed that the
disruption of microtubules affected the dynamics of IPS-1 and might dissociate its
interaction with vRCs and avSGs. I treated HeLa cells with nocodazole, which disrupts
the organization of microtubules. The nocodazole treatment in uninfected cells did not
induce significant changes in the morphology of mitochondria, as determined by
staining for FLAG-IPS-1 (Figure 3-12A, upper). In NDV-infected cells without the
treatment, FLAG-IPS-1 partially merged with both vRCs/avSGs as observed with the
yellow interface (Figure 3-12A, lower). The nocodazole treatment clearly impaired
these interactions, as judged by the disappearance of the interface. IFNB gene
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A
B
Figure 3-11. IPS-1 localization in NDV-infected cells
(A and B) FLAG-IPS-1/HeLa and HeLa cells were either mock treated or infected with
NDV (MOI = 1) for 6 and 12 hours. The cells were immunostained for FLAG (green in A),
IPS-1 (green in B), L (red), and TIAR (white). Nuclei were stained with DAPI (blue). The
white scale bar corresponds to 10 μm. The boxed area was enlarged and is displayed on the
right (zoom). Partial co-localization between IPS-1 and L (vRC) or TIAR (avSG) is shown
by displaying them in green and red, respectively (zoom).
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Figure 3-12. Nocodazole treatment for the inhibition of IPS-1 dynamics
A
B C
(A) FLAG-IPS-1/HeLa cells were mock treated (NT) or treated with 10 μg/μl of
nocodazole for 1 hour. After the treatment, the cells were mock treated or infected with
NDV (MOI = 1) for 12 hours and immunostained for FLAG (green), L (red), and TIAR
(white). Nuclei were stained with DAPI (blue). The white scale bar corresponds to 10 μm.
The two boxed areas in the merged image were enlarged and are shown on the right (zoom
1 and 2). (B and C) FLAG-IPS-1/HeLa cells were mock treated (NT) or treated with 0.625,
2.5, or 10 μg/μl of nocodazole for 1 hour followed by mock or NDV infection (MOI = 1)
for 12 hours. Expression levels of IFNB mRNA (B) and NDV-N RNA (C) were quantified
by RT-qPCR. Data is represented as means of ±SD (t-test: ***p<0.01, **p<0.05, *p<0.1,
NS = not significant). ND = not detectable.
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expression induced by NDV was dose-dependently impaired by nocodazole (Figure
3-12B), strongly suggesting that the observed interface was physiologically relevant.
Conversely, NDV replication was dose-dependently increased by nocodazole, thereby
excluding the possibility that the treatment attenuated the expression of IFNB by
inhibiting viral replication (Figure 3-12C). Takeing these observations together, I
hypothesized that RIG-I initially accumulated in NDV vRCs to undertake primary IFN
induction, and subsequently translocated to avSGs to boost a strong secondary IFN
production in the later phase. In each case, IPS-1 mediated RIG-I-triggered signaling by
closely associating with RIG-I within both granules.
3-8. avSGs were required for full activation of IFNB gene expression
To elucidate the significance of avSGs in IFNB gene expression further, I blocked
SGs formation. I knocked down the expression of PKR, which acts as a sensor for viral
infection and triggers avSG formation [58]. I also targeted G3BP proteins (G3BP1 and
G3BP2: G3BPs), which are critical for SG assembly and its maintenance [55, 63, 66]. I
knocked down RIG-I for comparison. A western blot analysis confirmed efficient
knockdown of target genes (Figure 3-13A). These cells were infected with NDV and
IFNB gene expression was examined by RT-qPCR (Figure 3-13B). As expected, the
knockdown of RIG-I impaired IFNB gene expression and the treatment with siPKR or
siG3BPs also inhibited IFNB gene induction. The kinetics of vRCs, avSGs, and IFNB
mRNA induction were subsequently examined in these cells (Figure 3-13C). These
results were quantified as in Figure 3-8 (Figure 3-14). In control cells at 12 hpi, vRCs
were observed in almost all cells (light gray + black, 97-99%) (Figure 3-14A).
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Knockdown of RIG-I, PKR or G3BPs did not decrease the number of vRCs-positive
cells (Figure 3-14B-D, light gray + black,). The knockdown of RIG-I markedly blocked
IFNB mRNA expression (red + green, 56.1 to 8.2% at 12 hpi) (Figure 3-15B); however,
the number of avSGs-positive cells remained constant (black, 91.4 to 83.8% at 12 hpi)
(Figure 3-14B). The knockdown of PKR strongly inhibited the formation of avSGs
(black, 91.4 to 17.9% at 12 hpi) (Figure 3-14C) and decreased the number of IFNB
mRNA-positive cells (red + green, 56.1 to 18.4% at 12 hpi) (Figure 3-15C). The
knockdown of G3BPs also inhibited avSGs (black, 91.4 to 21.7% at 12 hpi) (Figure
15D) and decreased the number of IFNB mRNA-positive cells (red + green, 56.1 to
21.8% at 12 hpi) (Figure 3-15D). Concomitant with the inhibition of avSGs, the number
of cells exhibiting vRCs and IFNB mRNA increased, suggesting that these cells failed
to develop into triple-positive, high IFNB gene-expressing cells (Figure 3-16C and D). I
also examined the expression of ISGs; ISG20, ISG56, and CXCL10 genes by RT-qPCR
at 12 hpi (Figure 3-16). The expression of these genes was dependent on RIG-I, PKR,
and G3BPs. These results strongly suggested that avSGs contributed to the robust IFNB
gene expression observed at approximately 12 hpi.
3-9. Viral poly(A)+ RNA stimulated IFNB induction
I observed that avSGs contained vRNA(+), but not vRNA(-) (Figure 3-3) nor
vdsRNA (Figure 3-5). I suspected that vRNA(+) activated RIG-I in avSGs, culminating
in IFNB gene expression at 12 hpi. vRNA(+) consists of viral mRNA (vmRNA)
containing 3’-poly(A) tail and viral anti-genomic complementary RNA (vcRNA), a
template RNA for vgRNA synthesis during viral replication. Since vmRNA is a major
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A B
C
Figure 3-13. Gene knockdown for the inhibition of avSGs formation
(A-C) HeLa cells were transfected with siRNAs; siCtrl, siRIG-I, siPKR, or siG3BPs (for
G3BP1 and G3BP2). After transfection, the cells were infected with NDV (MOI = 1) for
the indicated time. (A) Expression levels of the indicated proteins were analyzed by
western blotting. (B) IFNB mRNA expression was analyzed by RT-qPCR. Data is
represented as means of ±SD (t-test: ***p<0.01, **p<0.05, *p<0.1, NS = not significant).
(C) Cells were immunostained for eIF3η (green) and N (red). IFNB mRNA (white) was
detected by the RNA-FISH method. Nuclei were stained with DAPI (blue). Merge 1:
nuclei, eIF3η, N. Merge 2: nuclei, eIF3η, N, and IFNB mRNA. The white scale bar
corresponds to 20 μm).
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A B
C D
Figure 3-14. Quantification of vRCs/avSGs in avSG-blocked cells
(A-D) HeLa cells were transfected with siRNAs; siCtrl, siRIG-I, siPKR, or siG3BPs
(siG3BP1 and siG3BP2). After transfection, the cells were infected with NDV (MOI = 1)
for the indicated time up to 12 hours. Three hundred cells at each time point of infection
were counted by MetaMorph software. Cells were categorized according to the existence of
vRCs and avSGs as shown above. Percentages are indicated inside the data bar.
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Figure 3-15. Quantification of IFNB mRNA expression in avSGs-blocked cells
A B
D C
(A-D) HeLa cells were transfected with siRNAs; siCtrl, siRIG-I, siPKR, or siG3BPs
(siG3BP1 and siG3BP2). After transfection, the cells were infected with NDV (MOI = 1)
for the indicated time up to 12 hours. Three hundred cells at each time point of infection
were counted by MetaMorph software. Cells were categorized according to the existence of
vRCs, avSGs, and IFNB mRNA as shown above. Percentages are indicated inside the data
bar.
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A B
C
Figure 3-16. ISGs induction in avSGs-blocked cells
(A-C) HeLa cells were transfected with siRNAs; siCtrl, siRIG-I, siPKR, or siG3BPs
(siG3BP1 and siG3BP2). After transfection, the cells were mock treated or infected with
NDV (MOI = 1) for 12 hours. Expression levels of ISG20, ISG56, and CXCL10 mRNA
were measured by qRT-PCR. Data is represented as means of ±SD (t-test: ***p<0.01,
**p<0.05, *p<0.1, NS = not significant).
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part of RNA that constitutes vRNA(+), I considered whether NDV poly(A) +
vmRNA
could be a potent activator of IFNB gene expression in avSGs. To obtain vmRNA, I
isolated total RNA from NDV-infected HeLa cells and fractionated into poly(A)+ and
poly(A)- RNA populations using oligo(dT)-combined latex beads (Figure 3-17). These
RNA fractions were tested for the induction of Ifnb mRNA in mouse embryonic
fibroblasts (MEFs). RNA fractions extracted from uninfected cells did not activate the
Ifnb gene (Figure 3-18A), whereas both poly(A)+ and poly(A)
- RNA extracted from
NDV-infected cells exhibited strong Ifnb-inducing activity. In addition to Ifnb, Isg56
and Cxcl10 genes were also activated by these RNA fractions (Figure 3-19). I further
tested other viruses belonging to Mononegavirales, RSV (Figure 3-18B) and VSV
(Figure 3-18C). The results revealed that poly(A)+ RNA from RSV and VSV also
exhibited strong stimulatory activity, suggesting that poly(A)-containing viral
transcripts of the Mononegavirales viruses potentially activates antiviral innate
immunity.
3-10. NDV poly(A)+ RNA formed double-stranded, 5’-triphosphate structure
To further characterize the structural feature of the immunostimulatory NDV
poly(A)+ RNA, the poly(A)
+ RNA fraction from NDV-infected HeLa cells was treated
with RNase III, CIAP, or DNase I. Ifnb and ISGs induction were dramatically impaired
by RNase III or CIAP, but not by the DNase I treatment (Figure 3-20A and 3-21),
suggesting that the stimulatory activity resides in poly(A)+ RNA species which formed a
secondary structure and end-phosphate moieties, such as 5’-triphosphate. To examine
the presence of 5'-triphosphate in the stimulatory RNA, the NDV poly(A)+ RNA
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53
A B
Figure 3-17. Isolation of NDV poly(A)+ RNA
(A) NDV viral RNA (NDV-N) expression level in the mock and NDV-infected (MOI = 1)
HeLa total RNA was measured by RT-qPCR. Data is represented as means of ±SD. ND =
not detected. (B) Experimental scheme of poly(A)+ RNA isolation. Total RNA of
mock/NDV-infected (MOI = 1) HeLa cells was incubated with oligo(dT)-combined latex
beads. By centrifugation, supernatant and precipitate were separated for the poly(A)- and
poly(A)+ RNA fraction respectively.
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54
Figure 3-18. Ifnb activation by poly(A)+ viral RNAs from Mononegavirales
A
B C
(A-C) Total RNA of mock treated or NDV-infected (12 hpi, MOI = 1) HeLa cells (A),
RSV-infected (60 hpi, MOI = 1) HEp-2 cells (B), or VSV-infected (12 hpi, MOI = 1) HeLa
cells (C) was separated into the poly(A)- and poly(A)
+ RNA fractions by
oligo(dT)-combined latex beads and transfected into MEFs (1×105 cells were transfected
with 200 ng RNA). Ifnb mRNA expression levels were measured by RT-qPCR. Data is
represented as means of ±SD.
A B
Figure 3-19. ISGs induction by NDV poly(A)+ RNA
(A and B) MEFs (2×105 cells) were transfected with total, poly(A)
-, or poly(A)
+ RNA (200
ng) from mock/NDV-infected (MOI = 1) HeLa cells. Isg56 and Cxcl10 mRNA expression
levels were measured by RT-qPCR. Data is represented as means of ±SD.
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55
Figure 3-20. Ifnb-inducing activity of enzyme-treated NDV poly(A)+ RNA
A B
(A and B) The poly(A)+ RNA fraction from NDV-infected (12 hpi, MOI = 1) HeLa cells
was mock treated (NT) or treated with RNase III, DNase I, CIAP (D), or the 5’-capping
enzyme of Vaccinia virus (E) and then transfected to MEFs (1×105 cells were transfected
with 200 ng RNA). Ifnb mRNA expression levels were measured by RT-qPCR. Data is
represented as means of ±SD (t-test: ***p<0.01, NS = not significant).
A B
Figure 3-21. ISGs induction by NDV poly(A)+ viral RNA
(A and B) The poly(A)+ RNA fraction from NDV-infected (12 hpi, MOI = 1) HeLa cells
was mock treated (NT) or treated with RNase III, DNase I, CIAP (D), or the 5’-capping
enzyme of Vaccinia virus (E) and then transfected to MEFs (1×105 cells were transfected
with 200 ng RNA). Ifnb mRNA expression levels were measured by RT-qPCR. Data is
represented as means of ±SD (t-test: ***p<0.01, NS = not significant).
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fraction was further subjected to reaction with the capping enzyme of vaccinia virus,
which adds a cap to the 5'-triphosphate end. The capping reaction diminished the Ifnb
gene induction by the RNA (Figure 3-20B), therefore I speculated that the stimulatory
activity of NDV poly(A)+ RNA was from the remaining 5'-triphosphate-containing
RNA.
3-11. NDV produced read-through transcript
Transcription of NDV occurs in the order 3’-Le-N-P-M-F-HN-L-Tr-5’. The first
transcript Le possesses a 5'-triphosphate and is devoid of a 3’-poly(A). However,
transcripts for N, P, M, F, HN and L possess both a 5'-cap and 3’-poly(A). I analyzed
viral N mRNA by strand-specific northern blotting (Figure 3-22A). RNA extracted from
NDV-infected HeLa cells was fractionated into poly(A)+ and poly(A)
- populations.
Ethidium bromide staining revealed the virtual absence of ribosomal RNA in the
poly(A)+ RNA fraction, demonstrating successful fractionation (Figure 3-22A, bottom).
As expected, the northern analysis with a N-specific RNA probe revealed that N mRNA
(1.7 kilonucleotides, knt) was enriched in the poly(A)+ RNA fraction (Figure 3-22A,
top). Interestingly, this probe detected additional slow migrating RNA at 3.8 knt. I
suspected that this larger RNA was read-through N mRNA, an extended transcript
covering the N and P genes [67]. Indeed, many types of viruses belonging to
Mononegavirales have been shown to produce read-through transcripts [68-70].
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Figure 3-22. Strand-specific northern blotting for NDV poly(A)+ RNA
A B
(A) Total, poly(A)-, and poly(A)
+ RNA (each 250 ng) from mock treated or NDV-infected
(12 hpi, MOI = 1) HeLa cells were separated on a denaturing agarose gel. An ethidium
bromide (EtBr)-stained gel is shown at the bottom. vRNA(+) was detected by blotting with
an N-specific RNA probe (N(-)). Positions of N vmRNA (N(+)) and N-P read-through
RNA (N-P(+) RT) are shown. knt = kilo nucleotide. RNA probe and target vRNA(+) are
illustrated alongside the results. (B) The poly(A)+ RNA from mock treated or NDV-infected
(12 hpi, MOI = 1) HeLa cells was subjected to strand-specific northern blotting using a
leader-specific RNA probe (Le(-)). The position of Le-N read-through RNA is shown
(Le-N(+) RT). *non-specific band. The RNA probe and target vRNA(+) are illustrated
alongside the results.
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3-12. NDV, RSV, and VSV produced leader-containing read-through RNA
I further examined the presence of Le-N read-through transcript, because such an
RNA possesses both 5'-triphosphate and 3'-poly(A) [69]. To detect Le-N read-through
RNA, I performed strand-specific northern analysis using Le-specific RNA probe
(Figure 3-22B). Poly(A)+ RNA from NDV-infected cells exhibited 1.7 knt signal by this
probe. Le RNA (55 nt) was undetectable in this gel due to its small size. To quantify
Le-N read-through RNA, we used a protocol for strand-specific RT-qPCR [65]. This
method provided specific detection of the target RNAs (Figure 3-23A). First, I
quantified the positive-strand Le-N sequence (Figure 3-24A). As expected, the Le-N
was enriched in the poly(A)+ fraction. Next, I quantified the negative-strand Le-N
sequence to exclude the possibility that the stimulatory activity is from vmRNA
partially hybridized with negative-strand vdRNA (Figure 3-24B). The result revealed
that negative-strand Le-N sequence was hardly detectable in the poly(A)+ fraction,
excluding the possibility mentioned above. Interestingly, this technique demonstrated
similar read-through transcripts (Le-NS1, RSV; Le-N, VSV) in cells infected with RSV
and VSV (Figure 3-23B, C, and 3-25), suggesting common potential
immunostimulatory activity of Le-N read-through transcript produced by the
Mononegavirales viruses.
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A
B
C
Figure 3-23. Validation of strand-specific RT-qPCR
(A-C) 1010
copies of vgRNA isolated from the viral particles and in vitro-synthesized RNA
(IVT RNA) corresponding to vmRNA (N, NS1 and N for NDV, RSV and VSV,
respectively) and read-through RNA (Le-N, Le-NS1 and Le-N for NDV, RSV and VSV,
respectively) were subjected to strand-specific RT-qPCR (ssRT-qPCR) using specific
primer sets (Table 3-2, 3-3, and 3-4). Percentage of the RNA copies of each target RNA is
shown. Data is represented as means of ±SD. The results show the specificity of
ssRT-qPCR: the probe for vRNA(-) only detected vgRNA; the probe for Le-N/NS1(+)
read-through RNA selectively detected Le-N/NS1 RNA but not N mRNA.
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60
Figure 3-24. Strand-specific RT-qPCR for NDV poly(A)+ RNA
A B
(A and B) Total, poly(A)-, and poly(A)
+ RNA (each 200 ng) from mock treated or
NDV-infected (12 hpi, MOI = 1) HeLa cells were subjected to strand-specific
RT-qPCR (ssRT-qPCR) targeting Le-N(+) read-through RNA (A) or targeting vRNA(-)
(B). Primers used are listed in Table 3-2. Data is represented as means ±SD. Target
vRNAs in the assay are illustrated below.
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61
Figure 3-25. Strand-specific RT-qPCR analysis for RSV and VSV vRNAs
A B C
F E D
(A-F) Total, poly(A)-, and poly(A)
+ RNA from mock treated, RSV-infected (60 hpi, MOI =
1) HEp-2 cells (A-C), or VSV-infected (12 hpi, MOI = 1) HeLa cells (D-F) were subjected
to strand-specific RT-qPCR (ssRT-qPCR) targeting Le-NS1/N(-) as a portion of vgRNA,
NS1/N(+) vmRNA, and Le-NS1/N(+) read-through RNA with specific primer sets (Table
3-3 and 3-4). Data is represented as means of ±SD.
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Table 3-1. Primers used for in vitro transcription
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Table 3-2. Primer sets used for ssRT-qPCR targeting NDV vRNAs
Table 3-3. Primer sets used for ssRT-qPCR targeting RSV vRNAs
Table 3-4. Primer sets used for ssRT-qPCR targeting VSV vRNAs
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64
3-13. In vitro Le-N RNA induced IFNB gene expression and SGs formation
Finally, I synthesized the RNA corresponding to Le and the Le-N read-through RNA
in vitro, and examined their immunostimulatory activity (Figure 26A). The in vitro
products of Le-N read-through RNA with 3’-poly(A) as well as Le RNA were capable
of inducing IFNB gene activation. I also monitored Cy3-labeled Le-N read-through
RNA with 3’-poly(A) after transfection. I observed that this RNA induced the formation
of SGs containing G3BP1 and Cy3 signal, and partially co-localized with SGs (Figure
3-26B). Here, Cy3 signal did not completely coincided with SGs, presumably because
most Cy3-RNA resided within endosomes and a portion of the RNA escaped into the
cytoplasm and induced SGs. The results strongly suggest that viral Le-N read-through
transcript is quite capable of inducing avSGs formation followed by induction of IFNB
gene via RIG-I signaling.
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Figure 3-26. IFNB gene and SGs induction by in vitro-transcribed Le-N RNA
A
B
A
(A) RNA corresponding to NDV Le RNA (Le(+)) and Le-N read-through RNA
(Le-N(+)+poly(A)) were synthesized in vitro. These RNA preparations (200 ng) were
tested for IFNB gene expression by transfecting to FLAG-RIG-I/HeLa cells (2×105 cells).
IFNB mRNA expression levels were measured by RT-qPCR. Data is represented as means
of ±SD. (B) Cy3-labeled Le-N(+)+poly(A) (40 ng) was synthesized in vitro and transfected
into EGFP-G3BP1/HeLa cells (0.25×105 cells). After 6 hours, cells were treated with 10
μM chloroquine for 1 hour to enhance transfection efficiency. Cells were fixed and
visualized for EGFP-G3BP1 (green) and Cy3-labelled RNA (red). Enlarged images of
boxed areas are shown (zoom 1 and 2).
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4. Discussion
4-1. vRCs; a locale where RIG-I recognizes its authentic ligand, vgRNA
In this study, I aimed to figure out how RIG-I spatiotemporally encounters its RNA
ligands and participates in cellular stress response upon NDV infection, which provokes
a strong IFN response in mammalian cells. Immunostaining experiments and
quantification analysis clearly demonstrated that NDV infection induced early
generation of vRCs (1.5 hpi) and subsequent avSGs formation (7.5 hpi) (Figure 3-2, 3-7,
and 3-8). RNA-FISH detection for NDV viral RNAs and IFNB mRNA further revealed
that both NDV vRNA(-) and vRNA(+) localized in vRCs in the early phase (6 hpi)
(Figure 3-3), when IFNB mRNA expression was already induced without avSGs (Figure
3-7 and 3-8). Importantly, RIG-I was detected in these early-formed vRCs along with
IPS-1 aggregation (6 hpi), which is widely recognized as a hallmark of initiation of IFN
induction [11, 12, 71] (Figure 3-11A). NDV vRNA(-) detected in vRCs is none other
than vgRNA, which is considered to be a natural ligand of RIG-I [28]. Since purified
NDV vgRNA activated IFNB gene expression in the transfected cells, and this activity
was considerably abolished by RNase III and CIAP treatment (data not shown), NDV
vgRNA supposedly forms a 5’-triphosphate dsRNA structure that corresponds to the
well-studied archetype of RIG-I ligands [41, 44, 45, 49, 72]. It was obscure as to
whether or not, as well as how, RIG-I encounters nucleoprotein-encapsidated vgRNA,
as with NDV vRCs observed in this study. However, recent studies clearly demonstrated
that RIG-I is accessible to 5’-triphosphate-dsRNAs within the incoming nucleocapsids
of RNA viruses, such as IAV, VSV, Rift Valley fever virus, and La Crosse virus, to
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provoke IFNB gene expression even in the absence of viral transcription and replication
[73]. In the light of this information, this study provided ample evidence to define NDV
vgRNA within primal vRCs as an authentic ligand of RIG-I. Therefore, vRCs are a
primordial source of the immediate-early IFN production driven by the RIG-I/IPS1
signaling axis.
4-2. avSGs; a locale where RIG-I encounters sequestered viral poly(A)+ RNA
To correlate the RIG-I-driven IFN-producing signaling with cellular stress response
upon viral infection, I carefully investigated its kinetics along with avSGs formation. In
line with previous reports [58, 59], the knockdown experiments in this study clearly
explained the contribution of avSGs to substantial host IFN response against NDV
infection (Figure 3-13, 3-15, and 3-16). Importantly, I discovered that NDV vRNA(+)
that was initially derived from vRCs migrated to avSGs, and RIG-I also accumulated in
avSGs along with IPS-1 (Figure 3-10 and 3-11). Meanwhile, I faced a critical question
of whether NDV vRNA(+) induces IFNB gene expression via RIG-I, and the alternative
of vmRNA or vcRNA to identify vRNA(+) in avSGs. Early studies fully described the
physiological function of SGs as the regulatory machinery of host mRNA metabolism
during environmental stresses: SGs sequester mRNA into a “safe shelter” composed of
a variety of mRNA binding proteins [74]. In fact, many types of negative-strand RNA
viruses including NDV produce 5’-capped and 3’-polyadenylated vmRNA as their
transcripts, and no distinguishable signatures between viral and host mRNA have been
unveiled so far. Thus, it was natural to presume that NDV poly(A)+ vmRNA was
internalized in avSGs together with host mRNA. I demonstrated that the poly(A)+ RNA
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fraction from NDV-infected HeLa cells contained NDV vmRNA, and was capable of
stimulating IFNB gene expression and concomitant ISGs induction in MEFs (Figure
3-18, 3-19, and 3-22). However, host mRNA generally conforms to a certain format;
therefore, it is not sensed as non-self. Since mRNA with m7G-cap is not sensed by
RIG-I, the acquisition of a cap is also a major strategy of viruses to escape immune
detection. The importance of additional methylation adjacent to the cap has recently
been reported [75, 76]. According to this theory, I suspected the presence of a novel
immunostimulatory vmRNA species in avSGs. Intriguingly, treatment of NDV poly(A)+
RNA fraction with RNase III, CIAP, and 5’-capping enzyme all resulted in significant
loss of the ability to induce IFNB gene and ISGs expression (Figure 3-20 and 3-21).
Therefore, it was convincing to conclude that NDV poly(A)+ vmRNA within avSGs
forms double-stranded and 5’-triphosphate structure, an eligible signature for an RIG-I
ligand, and activated RIG-I and IFNB gene expression.
4-3. avSGs; a critical platform for the RIG-I sensing of multiple viral RNAs
Nonetheless, the viral RNA composition of avSGs observed in this study was
different from IAV-induced avSGs, in which vgRNA was shown to be localized [58].
Moreover, avSGs induced by EMCV infection were previously reported to contained
vdsRNA [63]. These findings indicate that the mechanism underlying the induction of
avSGs and the viral RNA content of avSGs is dependent upon virus types. In any of
these cases, however, avSGs play a critical role in the host antiviral IFN response. PKR
is known to be essential for the formation of avSGs induced by NDV (Figure 3-13) and
other viruses [56]. In measles virus (MeV) infection, PKR was shown to correlate
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70
positively with both avSGs formation and IFN expression [77-79]. DHX36, another
DExD/H-box helicase, has been shown to cooperate with PKR for the formation of
avSGs and subsequent viral RNA sensing by RIG-I [59]. Pumilio proteins (PUM1 and
2) are known to cooperate with LGP2 to sense vRNA in order to trigger IFN-inducing
signaling [80]. PKR, DHX36, and PUM1/2 are specifically recruited to avSGs. A
critical signaling molecule for IFN induction, TRIM25, was shown to be specifically
recruited to avSGs [59]. Taken together, these findings and the results from my study
demonstrated that avSGs serve as a platform for the efficient sensing of viral RNA
through the recruitment of antiviral molecules and critical signaling molecules.
4-4. The viral read-through transcripts; a novel natural ligand of RIG-I
In the last part of this study, I revealed that NDV, as well as other viruses belonging
to Mononegavirales including RSV and VSV, produced Le-N/NS1 read-through RNA
with 3’-poly(A) (Figure 3-22B, 3-24A, 3-25C, and F). The Le transcript has been shown
to contain 5’-triphosphate [61, 81], and synthetic RNA corresponding to the Le-N
activated IFNB gene (Figure 3-26A). These results strongly suggest that viral Le-N
RNA synthesized within vRCs was transported to avSGs and detected by RIG-I. The
generation of uncapped vRNA through transcriptional read-through is commonly found
in other Mononegavirales viruses such as SeV, MeV, and VSV [68-70], suggesting that
sensing of such uncapped viral RNA is an important strategy of antiviral innate
immunity against these viruses. In the case of MeV, 5’-triphosphate Le RNA is
considered to be a signature of activating RIG-I for IFN induction, and the association
of RIG-I with the 5’ end of the transcript covers the Le to N regions, corresponding to
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the Le-N read-through transcript [82]. In future studies, it will be interesting to explore
the general impact of an abortive Le-N read-through transcript, which is exclusively
produced by viruses belonging to Mononegavirales, on the RIG-I-driven IFN pathway.
4-5. Conclusion
In this study, I intensively explored the association of RIG-I-driven antiviral IFN
response with clusters of RNP complexes, vRCs and avSGs, in which potential RIG-I
ligands exist. Although I entirely employed NDV for the most part in my experiments, I
can suggest an important model to interpret the absolute function of RIG-I on the viral
RNA detection and integration of the antiviral IFN response and stress response in the
viral and host RNP complexes (Figure 4-1). The virus sensing in the cytoplasm involves
more than simple interaction between sensor molecules and viral RNAs; a more
complex mechanism, including various RNA binding proteins and stress response
machinery, is responsible for detecting various viral RNA structures generated by the
replication of different viruses. Thus, it will be interesting to investigate other types of
viruses that deploy their own replication sites and avSGs to understand more about the
fundamental role of RIG-I on the IFN response against viral infections.
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Figure 4-1. Kinetic model of NDV viral RNA sensing of RIG-I
NDV generates viral replication complexes (vRCs) in the early phase. RIG-I initially
detects vgRNA(-) within early vRCs and induces primary IFN production in
collaboration with IPS-1 aggregated around vRCs. At viral transcription, vmRNA and
5’-uncapped, 3’-polyadenylated read-through RNA (RT RNA) are produced. In the
later phase, vdsRNA is produced as a replicative intermediate, forming the duplex of
vgRNA(-) and vcRNA(+). PKR is activated by this product, or possibly by vgRNA or
RT RNA with a double-strand in early vRCs. Once avSGs are induced,
poly(A)-containing vmRNA and RT RNA are captured in avSGs, as well as host
mRNAs. Subsequently, RIG-I accumulates in avSGs to detect 5’-uncapped RT RNA
with IPS-1, and induce a secondary burst of IFN.
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6. Acknowledgements
Firstly, I would like to express my sincere gratitude to my advisor Prof. Takashi
Fujita for his continuous support of my Ph.D. studies, for his patience, motivation, and
immense knowledge. His guidance helped me at all times during my research. As well
as my advisor, I would like to thank Assoc. Prof. Hiroki Kato for his insightful
comments and encouragement, and also for his strict guidance which incented me to
widen my research to consider perspectives. My sincere thanks also goes to Prof.
Mitsutoshi Yoneyama, Dr. Koji Onomoto, and Dr. Kazuhide Onoguchi; their advice on
both my research as well as on my career has been priceless. I would also especially like
to thank Assoc. Prof. Takahiro Fujiwara and Dr. Fumiyoshi Ishidate for the exciting
work on the imaging experiments. I thank my lab members for the stimulating
discussions, for the hard-working sleepless nights, and for all the fun moments we have
shared together.
A special thanks to my family. Words cannot express how grateful I am to my father,
my mother, my sister, and my wife. All of your supports, sacrifices, encouragements,
and love offered mental and spiritual sustenance for me.
This thesis was written based on the following original article;
Seong-Wook Oh, Koji Onomoto, Mai Wakimoto, Kazuhide Onoguchi, Fumiyoshi Ishidate, Takahiro
Fujiwara, Mitsutoshi Yoneyama, Hiroki Kato, and Takashi Fujita
“Leader-Containing Uncapped Viral Transcript Activates RIG-I in Antiviral Stress Granules”
PLOS Pathogens 12(2), e1005444 (2016)
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